Add estimatedPlayoutTimestamp to RTCInboundRTPStreamStats.

https://w3c.github.io/webrtc-stats/#dom-rtcinboundrtpstreamstats-estimatedplayouttimestamp

Partial implementation: currently only populated when a/v sync is enabled.

Bug: webrtc:7065
Change-Id: I8595cc848d080d7c3bef152462a9becf0e5a2196
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/155621
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29581}
diff --git a/call/video_receive_stream.h b/call/video_receive_stream.h
index 6e08738..cff8126 100644
--- a/call/video_receive_stream.h
+++ b/call/video_receive_stream.h
@@ -110,6 +110,8 @@
 
     VideoContentType content_type = VideoContentType::UNSPECIFIED;
 
+    // https://w3c.github.io/webrtc-stats/#dom-rtcinboundrtpstreamstats-estimatedplayouttimestamp
+    absl::optional<int64_t> estimated_playout_ntp_timestamp_ms;
     int sync_offset_ms = std::numeric_limits<int>::max();
 
     uint32_t ssrc = 0;